Fission vs. Fusion – What’s the Difference?

Editor's note: This article was originally published on January 30, 2013. It has been revised, updated and republished. 

sun
NASA
Inside the sun, fusion reactions take place at very high temperatures and enormous gravitational pressures

Look up during the day to see one of the most powerful examples of a nuclear reactor: the sun. Inside the sun, fusion reactions take place at very high temperatures and enormous gravitational pressures.

The foundation of nuclear energy is harnessing the power of atoms by splitting apart, a process called fission, or combining them, called fusion. Both fission and fusion alter atoms to create energy, but what is the difference between the two?

Fission, a term coined by scientists LIse Meitner and Otto Frisch, is named after the term “binary fission” in biology to describe cell division. Just as cell’s divide, in fission an atom splits into smaller particles. Fission takes place when a large, somewhatunstable isotope (atoms with the same number of protons but different number of neutrons) is bombarded by high-speed particles, usually neutrons. These neutrons are accelerated and then slammed into the unstable isotope, causing it to fission, or break into smaller particles. During the process, a neutron is accelerated and strikes the target nucleus, which in the majority of nuclear power reactors today is Uranium-235. This splits the target nucleus and breaks it down into two smaller isotopes (the fission products), three high-speed neutrons, and a large amount of energy. This resulting energy is then used to heat water in nuclear reactors and ultimately produces electricity. The high-speed neutrons that are ejected become projectiles that initiate other fission reactions, or chain reactions.

fission
EIA
Nuclear Fission

Conversely, fusion takes place when two low-mass isotopes, typically isotopes of hydrogen, unite under conditions of extreme pressure and temperature. Atoms of Tritium and Deuterium (isotopes of hydrogen, Hydrogen-3 and Hydrogen-2, respectively) unite under extreme pressure and temperature to produce a neutron and a helium isotope. Along with this, an enormous amount of energy is released, which is several times the amount produced from fission.

fusion
DOE
Nuclear Fusion

While fission is used in nuclear power reactors since it can be controlled, fusion is not yet utilized to produce power. Some scientists believe there are opportunities to do so. Fusion offers an appealing opportunity, since fusion creates less radioactive material than fission and has a nearly unlimited fuel supply. These benefits are countered by the difficulty in harnessing fusion. Fusion reactions are not easily controlled, and it is expensive to create the needed conditions for a fusion reaction. However, research continues into ways to better harness the power of fusion, but research is in experimental stages, as scientists continue to work on controlling nuclear fusion in an effort to make a fusion reactor to produce electricity.

Both fission and fusion are nuclear reactions that produce energy, but the processes are very different. Fission is the splitting of a heavy, unstable nucleus into two lighter nuclei, and fusion is the process where two light nuclei combine together releasing vast amounts of energy. While different, the two processes have an important role in the past, present and future of energy creation.

Comments (8)

Posted November 15, 2021 by Edward Brady
SMR - Small modular Reactors https://www.nuscalepower.com/benefits/smallest-reactor
Posted November 09, 2021 by George Bombardier
Aren't we still just boiling water to drive turbines? We need to figure out a way to control fusion in a continuous bust similar to a white star only in miniature size, then surround it with solar panels to convert that power to electricity.
Posted October 21, 2021 by Tony Wallace
As an old thermodynamics engineer I have always subscribed to the energy balance principle that you can never get something for nothing. Yet her we appear to have two opposite processes; one which combines two light particles to form a heavier particle, and the other which splits a heavy particle into two lighter particles; and both give off vast amounts of energy. This appears to me to be the perfect perpetual motion situation; which I have always believed was impossible to achieve. So, my question is where did all the energy that these two processes are releasing come from in the first place?
Posted October 19, 2021 by Theodore Silver
It is my (rudimentary) understanding, as a general proposition, that when one or more relatively unstable species are subjected to such processes as one or more relatively stabler species, energy is released (the process is "exothermic.") Concomitantly if one or more relatively stabler species are subjected to such processes as produce one or more relatively unstable species, energy is consumed (the process is "endothermic.") Mindful of that proposition (if accurate), I am at a loss to understand how it is that (1) disrupting an intact nucleus (fission) releases energy (exothermia), and (2) creating new nuclei (fusion) also releases energy (exothermia). In my ignorance, I'm not understanding how to processes each (seeming to be) the converse of the other -- both release energy. Can you help me?
Posted October 05, 2021 by Charles
These Mathematics are thoroughly confusing, but it looks like you know what you're saying so ok. What happens if fusion astronomy alludes to the fascinating work of herpetology which changes my hypothesis of the art of breaking down an atom?
Posted September 16, 2021 by learn
YOU spelled wrong LIse Meitner. You wrote lisa
Posted September 09, 2021 by Andy Sefton
Fascinating - so well explained.
Posted August 30, 2021 by michael lombardi
i like it. this is very interesting to me.

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